Comparative study on the physicochemical characteristics of lignin via sequential solvent fractionation of ethanol and Kraft lignin derived from poplar and their applications

The challenges in high value-added utilization of lignin own to its wide molecular weight distribution and complex physicochemical properties associated with it. How to narrow the molecular weight of lignin and elucidate the differences in physiochemical properties related to molecular weight of lignins is of crucial importance for lignin conversion processing. In this study, lab-made Kraft and ethanol lignins were fractionated into several narrower molecular weight distribution components through a designed solvent system. The yield, molecular weight distribution, structural characterization and their physicochemical properties of each fractionated lignin were analyzed comparatively. The results indicated that the yield of ethanol lignin is higher (8.44 %) than that of Kraft lignin (5.26 %). Kraft lignin shows lower molecular weight (Mw=2480 g/mol), higher hydroxyl content (4.29 mmol/g), and lower content of β- O -4 and β-5 bonds compared to ethanol lignin (Mw=3682 g/mol, 2.10 mmol/g). Meanwhile, the phenolic hydroxyl content in both ethanol and Kraft lignins increased with the decrease of molecular weight. Furthermore, ethanol lignin demonstrated higher antioxidant activity and much higher (65times) tensile strength of lignin-based polyurethane elastomer than that of Kraft lignin-based polyurethane elastomer.